The present invention generally relates to the illumination of roadways at night. More specifically, the present invention relates to the use of solid-state light sources in roadway lighting as a way to improve over state of the art roadway lighting.
The adequate illumination of roadways is known to be problematic, particularly because of the competing interests in designing roadway lighting systems. For example, one usually seeks to minimize the cost of a lighting system (both capital and operating) but must also consider such things as glare, lighting uniformity, and other parameters which are well known in the art and regulated by the industry; British Standard (BS) 5489-1:2003 and Illuminating Engineering Society (IES) RP-8-00 are but two examples of the standards and codes which govern roadway lighting design.
To better understand the conflicting design interests, some discussion of how roadway lighting impacts a driver is warranted. As is well known, a roadway is typically illuminated by a plurality of light fixtures elevated and generally positioned above the roadway by a support structure (also referred to as a pole). Each fixture typically includes a single light source (e.g., metal halide (MH) or high pressure sodium (HPS)) and some form of optic (e.g., reflector, visor, lens, etc.) to provide limited control of the light projected therefrom. Each elevated fixture is aimed generally downward and located a significant distance from the next elevated fixture (e.g., on the order of seven times the mounting height) so to illuminate a significant stretch of road (also referred to as a carriageway).
As is well known, a night-time driver has specific vision requirements to ensure maximum visibility and driver safety. The typical night-time driver requires a minimum luminous intensity to perform the visual task (e.g., perceive objects on the roadway, respond to signage, maneuver between lanes, etc.) and a minimum contrast to distinguish between objects and the background; this is particularly true for objects in a driver's periphery as a night-time driver is (i) less able to perceive contrast than a daytime driver and (ii) less able to perceive contrast for objects that are not in detail (objects in one's periphery typically being in less focus than objects directly in one's line of sight).
So as an example, the cost of a roadway lighting system could be reduced by reducing the mounting height of the lighting fixtures; however, if the fixtures are lowered enough that a driver can directly see the light source, the perceived brightness of the light source can diminish the driver's ability to perceive contrast and, if the driver views the light source for a prolonged period, cause the driver's eyes to adapt to the brighter source and thus, have insufficient luminous intensity (also referred to as illumination or light level) for the visual task.
As another example, adding additional poles with corresponding fixtures to a roadway lighting system could reduce or eliminate areas of insufficient luminous intensity (i.e., improve the lighting of poorly-lit areas), thereby improving lighting uniformity and driver visibility; however, this would greatly increase the cost of the lighting system as it is well known that the cost of a support structure can equal that of the fixture itself, if not exceed the cost of the fixture. Alternatively, additional fixtures could be added to the existing poles and aimed at portions of the roadway with lower light levels; however, adding fixtures to an existing pole is no simple task. Since traditional light sources are large and the fixtures are aimed generally downward, introduction of additional fixtures aimed at various angles requires large visors to provide a desired cutoff so to prevent glare and other adverse lighting conditions already described. Care must be taken so that one fixture's cutoff does not adversely affect another fixture's cutoff. Further, a pole must be substantial enough to both provide the surface area for affixing the new fixtures and withstand anticipated wind loading.
Of course, some roadway lighting applications cannot make use of large support structures. For example, on bridges and in tunnels fixtures are typically mounted to walls, dividers, medians, or other existing structural features at or near the estimated eye height of a driver. Said fixtures cannot be aimed generally downward and provide adequate illumination of the roadway and so are aimed at a shallow angle; however, the aiming angle must be carefully selected so to avoid being directly viewable by the driver (the dangers of which have been described). One proposed solution is to combine the low mounting height with directional illumination (e.g., fixtures aimed so to project light in the direction a vehicle is traveling). One example is the MIRTRAN™ system commercially available from MUSCO® Lighting of Oskaloosa, Iowa, USA and commonly used to illuminate racetracks; see also U.S. Pat. Nos. 5,402,327, 5,647,661, and 6,220,726. Systems like MIRTRAN™ meet the strict needs of racecar driving (e.g., very high speeds, color and light levels in accordance with television broadcast requirements, etc.) but are somewhat overspecialized for traditional roadway lighting; further, systems like MIRTRAN™ still employ a single, large light source with limited control of the light projected therefrom.
So it can be seen that the current state of the art of roadway lighting is limited by conflicting design factors. The art would benefit from apparatus and methods for adequately illuminating a roadway in a manner that does not adversely affect a driver's visibility but also maintains cost-effectiveness. Thus, there is room for improvement in the art.